I think the confusion here is also rooted at the fact that photos and videos in the Photos library are not simply files copied to storage. Photos also stores me

I think the confusion here is also rooted at the fact that photos and videos in the Photos library are not simply files copied to storage. Photos also stores metadata about edits, renditions and thumbnails and other data needed for various functions of the Photos app. So when you’re syncing photos to iCloud, it’s not just the individual files that get synced but it’s the “Photos Library” managed container of the Photos app. If you add individual files directly in Finder or the Files app then their size matches exactly both in iCloud and on the local file system.